1. Field of the Invention
The present invention relates to a device and a method for manufacturing an optical preform by means of an internal vapour deposition process, said device comprising an energy source and a substrate tube which substrate tube comprises a supply side for supplying glass-forming precursors and a discharge side for discharging constituents that have not been deposited on the interior of the substrate tube, said energy source being movable along the length of the substrate tube between a point of reversal at the supply side and a point of reversal at the discharge side.
2. Background
Such a device is known per se from Korean patent application No. 2003-774952. The device that is known therefrom is used for manufacturing an optical preform by means of an MCVD (Modified Chemical Vapour Deposition) process, wherein use is made of a discharge tube and an insertion tube, which discharge tube is attached to the substrate tube. The insertion tube is positioned in the discharge tube and has a diameter that is smaller than that of the discharge tube. Disposed inside the insertion tube is an element for scraping off soot, comprising a bar which rotates in the interior of the insertion tube and which is in contact with the interior thereof. An annular space is present between the insertion tube and the discharge tube, through which annular space gases are passed.
From International application WO 89/102419 a device for manufacturing an optical preform by means of an internal vapour deposition process is known, wherein a tubular portion is mounted to the pump side of a substrate tube for removing solid undeposited particles. Such a device in particular comprises a screw structure that follows the internal surface of the tubular portion, which screw structure comprises an open, rotatable gas conduit wound in the form of a spiral.
During the deposition of glass layers, which may or may not be doped, in the interior of a substrate tube, in particular by means of a PCVD (Plasma Chemical Vapour Deposition) process, layers of low-quality quartz may be deposited, in particular in the area located outside the range of the reciprocating movement of the energy source, viz. the resonator, along the length of the substrate tube. Examples of such low-quality quartz layers include so-called soot rings, for example but also quartz with a high internal stress level caused by a high dopant content. The present inventors have found that such low-quality quartz may adversely affect the substrate tube, in particular as a result of the formation of gas bubbles at the supply side of the substrate tube upon contraction of the hollow substrate tube into a solid form. In addition to that the present inventors have found that such low-quality quartz may come loose from the substrate tube during the contraction process, which may result in contamination or bubble formation elsewhere in the substrate tube. Another negative aspect is that cracks may occur in the low-quality quartz region, which cracks may propagate in the direction of the centre of the substrate tube, which is undesirable. The present inventors have furthermore found that the low-quality quartz may lead to clogging, as a result of which the pressure may rise to an undesirably high level during the deposition process, which has a negative effect on the deposition process in the substrate tube, which manifests itself in the form of a white colour in practice.
The substrate tube is made of high-quality quartz. In practice the total length of the substrate tube will be longer than the portion of the substrate tube that wilt eventually be converted into a glass fibre by means of a drawing process, however, because the two ends of the substrate tube, where deposition takes place, may cause undesirable side effects, viz. deposition defects, contamination, bubble formation and the like.
Thus it is an object of the present invention to provide a device for manufacturing an optical preform by means of an internal vapour deposition process wherein the aforesaid problems, Which can be attributed mainly to the low quality of the quartz, are eliminated.
Another object of the present invention is to provide a device for manufacturing an optical preform by means of an internal vapour deposition process wherein no bubble formation or other undesirable effects occur during the contraction process, when the hollow substrate tube is converted into a solid preform.